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THEORETICALLY, the shoot apex of higher plants that length rooted only in light. Large plants were should prove the ideal object for experimental mor-grown from these cultures. Smith (1940) grew stem phology. It produces by its activities tissues from tips (0.55 to 4.0 mm, long) of Helianthus annuus which all the vegetative parts of the shoot are dif-on agar in White's solution. Studies were made of ferentiated. The problems of differentiation, or ma-the effects of various accessory growth factors in turation of tissues down the plant shoot, have been the medium on the rates of growth. Loo (1945) subjected to few physiological studies. Higher grew stem tips (5 to 10 mm, long) of Asparagus plants, with the young, embryonic tissues at the officinalis in sterile culture through twenty successtem tip, and progressively more differentiated or sive transfers. He considered these cultures to be mature tissues in the subjacent regions, are excel-the first true stem (organ) cultures that have been lent material for in vitro studies.achieved and to be comparable with the organ cul-Little consideration in the published work has tures of excised roots described by White, Robbins, been given to such significant morphological char-Bonner and others. De Ropp (1945) reported that acteristics as polarity, potentiality for development the growth of excised stem tips of rye embryos was of tissues during maturation, and correlation. The confined to the first leaf. When roots were produced, great theoretical interest of such data prompted the which was rarely, growth was more normal. writer to carry out several series of sterile cultures Much controversy has occurred in the literature of shoot apices (with little additional tissues), and concerning the capacity for growth and developof subjacent tissues undergoing maturation. ment of vegetative plant cells. It was the purpose Rechinger (1893) grew isolated buds of Populus of the writer to obtain evidence on this matter by nigra, Fraeinus ornus and other plants in sand with-comparing the growth in vitro of the apical, meriout nutrient materials. Some basal callus and a stematic tissues with that of the subjacent maturslight expansion of upper parts occurred, but no ing tissues of the shoot. roots were formed. Goebel (1908) grew isolated Also, since the respiratory rates of the tissue buds of Ceratopteris thalictroides in moist peat and pieces used in these experiments had been previfound that they produced leaves much smaller and ously determined (Ball and Boell, 1944), it was simpler morphologically than those of normal considered worth while to find whether a relationplants. Robbins (1922) apparently first grew shoot ship exists between rate of respiration and developtips in sterile conditions with sugar. He grew stem mental capacity. tips (1.75 to 3.75 em, long) of pea, corn and cotton Further, these cultures lent themselves well to in the dark in agar with glucose. The tips produced studies of the polarity of the buds and subjacent shoots with small chlorotic leaves and numerous...

The rash in PEP typically begins in the abdominal striae with sparing of the umbilicus. Spreading to the trunk and extremities may occur over a period of days but the face, palms and soles are usually unaffected. The pruritic papules may coalesce to form urticated plaques. Subepidermal vesicle formation has been described in PEP, 5 but not frank bullae. The condition is most unlikely to recur 6 and is not associated with an adverse outcome for either the mother or the infant. 1,6 Treatment 7 involves reassurance of the mother of the benign nature of the condition and the use of topical, moderately potent steroids and soothing emollients. A short course of systemic steroids may be required for severe cases.The histology of the condition is relatively characteristic but not specific. Several other conditions can give similar histological appearances including PG, allergic contact dermatitis, and drug or arthropod reactions. Therefore clinicopathological correlation is essential. The clinical differential diagnosis of PEP includes viral exanthemas, drug eruption, erythema multiforme, urticaria, contact dermatitis and early PG. At the time of their onset PEP and PG may be difficult to distinguish on clinical and histological grounds. 7 The development of bullous lesions has previously been considered diagnostic of PG. However, apart from the development of bullous lesions, this case had all of the hallmarks of PEP. The two disorders can be confidently differentiated by direct immunofluorescence of perilesional skin which demonstrates complement deposition at the basement membrane zone in all cases of PG and is consistently negative in PEP. The absence of complement deposition at the basement membrane has been confirmed by immunoelectron microscopy. 8 Others have reported unusually florid cases of PEP with features that also raised the possibility of PG, such as the presence of vesicles, involvement of palms and soles and continuation of the rash into the postpartum period. 9 The development of frankly bullous lesions would, until now, have been regarded as indicative of PG. However, this patient demonstrates that bullous lesions can also occur in PEP. Therefore this case redefines our clinical criteria for the diagnosis of pregnancy eruptions and highlights the crucial role of immunofluorescence studies in their accurate diagnosis.

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